XYZs of Signal Sources
The Complete Measurement System · · · 5
The Signal Source · · · 6
Analog or Digital? · · · 7
Basic Signal Source Applications · · · 8
Verification · · · 8
Analyzing Digital Modulation · · · 8
Characterization · · · 8
Testing D/A and A/D Converters · · · 8
Stress/Margin Testing · · · 9
Stressing Communication Receivers · · · 9
Signal Generation Techniques · · · 9
Understanding Waveforms · · · 10
Waveform Characteristics · · · 10
Amplitude, Frequency, and Phase · · · 10
Rise and Fall Time · · · 10
Pulse Width · · · 11
Offset · · · 12
Differential vs. Single-ended Signals · · · 12
Basic Waves · · · 13
Sine Waves · · · 13
Square And Rectangular Waves · · · 13
Sawtooth and Triangle Waves · · · 14
Step and Pulse Shapes · · · 14
Complex Waves · · · 15
Signal Modulation · · · 15
Analog Modulation · · · 15
Digital Modulation · · · 15
Quadrature Modulation · · · 16
Digital Patterns and Formats · · · 16
Bit Streams · · · 17
Contents
XYZs of Signal Sources
Types of Signal Sources · · · 17
Analog and Mixed Signal Sources · · · 18
Types of Analog and Mixed Signal Sources · · · 18
Arbitrary Generators · · · 18
Arbitrary/Function Generator (AFG) · · · 18
Arbitrary Waveform Generator (AWG) · · · 20
The Systems and Controls of a Mixed Signal Source · · · 22
Performance Terms and Considerations · · · 24
Memory Depth (Record Length) · · · 24
Sample (Clock) Rate · · · 24
Bandwidth · · · 26
Vertical (Amplitude) Resolution · · · 26
Horizontal (Timing) Resolution · · · 27
Region Shift · · · 27
Output Channels · · · 28
Digital Outputs · · · 28
Filtering · · · 29
Sequencing · · · 29
Integrated Editors · · · 31
Data Import Functions · · · 31
Built-in Applications · · · 32
Creating Waveforms Using Mixed Signal Sources · · · 33
Creating Waveforms Using ArbExpress
™· · · 34
Logic Signal Sources · · · 35
Types of Logic Signal Sources · · · 35
Pulse Pattern Generator (PPG) · · · 35
Data Timing Generator (DTG) · · · 35
The Systems and Controls of a Logic Signal Source · · · 40
Performance Terms and Considerations · · · 41
Data Rate · · · 41
Pattern Depth · · · 41
Rise/Fall Time · · · 41
Vertical (Amplitude) Resolution · · · 41
Horizontal (Timing) Resolution · · · 41
Output Channels · · · 42
Sequencing · · · 42
Integrated Editors · · · 42
Data Import Functions · · · 42
Creating Waveforms Using a Logic Signal Source · · · 43
Conclusion · · · 44
The Complete Measurement System
An acquisition instrument – usually an oscilloscope or logic analyzer – is probably the first thing that comes to mind when you think about making electronic measurements. But these tools can only make a measurement when they are able to acquire a signal of some kind. And there are many instances in which no such signal is available unless it is externally provided.
A strain gauge amplifier, for example, does not produce sig- nals; it merely increases the power of the signals it receives from a sensor. Similarly, a multiplexer on a digital address bus does not originate signals; it directs signal traffic from counters, registers, and other elements. But inevitably it becomes neces- sary to test the amplifier or multiplexer before it is connected to the circuit that feeds it. In order to use an acquisition instru- ment to measure the behavior of such devices, you must provide a stimulus signal at the input.
To cite another example, engineers must characterize their emerging designs to ensure that the new hardware meets design specifications across the full range of operation and beyond. This is known as margin or limit testing. It is a meas- urement task that requires a complete solution; one that can generate signals as well as make measurements. The toolset for digital design characterization differs from its counterpart in analog/mixed signal design, but both must include stimulus instruments and acquisition instruments.
The signal source, or signal generator, is the stimulus source that pairs with an acquisition instrument to create the two elements of a complete measurement solution. The two tools flank the input and output terminals of the device-under-test (DUT) as shown in Figure 1. In its various configurations, the signal source can provide stimulus signals in the form of ana- log waveforms, digital data patterns, modulation, intentional distortion, noise, and more. To make effective design, charac- terization, or troubleshooting measurements, it is important to consider both elements of the solution.
Figure 1. Most measurements require a solution made up of a signal source paired with an acquisition instrument. Triggering connectivity simplifies capturing of
the DUT output signal.
The purpose of this document is to explain signal sources, their contribution to the measurement solution as a whole, and their applications. Understanding the many types of signal sources and their capabilities is essential to your work as a researcher, engineer, or technician. Selecting the right tool will make your job easier and will help you produce fast, reliable results.
After reading this primer, you will be able to:
Describe how signal sources work Describe electrical waveform types
Describe the differences between mixed signal sources and logic signal sources
Understand basic signal source controls Generate simple waveforms
Should you need additional assistance, or have any comments or questions about the material in this primer, simply contact your Tektronix representative, or visit www.tektronix.com/
signal_sources.
The Signal Source
The signal source is exactly what its name implies: a source of signals used as a stimulus for electronic measurements. Most circuits require some type of input signal whose amplitude varies over time. The signal may be a true bipolar AC
1signal (with peaks oscillating above and below a ground reference point) or it may vary over a range of DC offset voltages, either positive or negative. It may be a sine wave or other analog function, a digital pulse, a binary pattern, or a purely arbitrary wave shape.
The signal source can provide “ideal” waveforms or it may add known, repeatable amounts and types of distortion (or errors) to the signal it delivers. See Figure 2. This characteristic is one of the signal source’s greatest virtues, since it is often impossible to create predictable distortion exactly when and where it’s need- ed using only the circuit itself. The response of the DUT in the presence of these distorted signals reveals its ability to handle stresses that fall outside the normal performance envelope.
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